A variety of carburetors are used to supply fuel to general purpose two-cycle engines. These engines are typically used as a source of motive power in small vehicles and portable machinery used in agriculture and forestry, etc. One particular type of carburetor has a structure in which a cylindrical throttle valve is installed crosswise in the air intake passage of the carburetor main body. The cylindrical throttle valve includes a throttle orifice and a metering valve, and is caused to move along its own central axial line while rotating in accordance with the operation of the accelerator pedal. The throttle valve controls the air flow rate by varying the degree of overlap of the throttle orifice with the air intake passage, and controls the fuel flow rate by varying the depth of insertion of the metering needle into the fuel nozzle. See, for example, in Japanese Patent Application Kokai No. Sho 58-101253 and Japanese Utility Model Application Kokai No. Sho 62-20158.
In a state where the accelerator pedal is released, the throttle valve is placed in a position which supplies the air and fuel necessary for idle revolution of the engine. From the idle position, the throttle valve is moved in accordance with the operation of the accelerator to increase the amounts of air and fuel.
As is universally known, the starting of an engine, especially starting at low temperatures, requires larger amounts of air and fuel than ordinary idling. Accordingly, in the aforementioned rotary throttle valve type carburetor, the throttle valve in the idle position is caused to move slightly by the operation of the accelerator so that the amounts of air and fuel are increased.
However, manual operation of the accelerator tends to result in excesses or insufficiencies in the amount of air and fuel supplied to the engine. Such excesses or insufficiencies will commonly cause the engine to fail to start. Thus, skill is required in order to obtain reliable starting. In addition, manual operation of the accelerator is extremely inconvenient because the state of operation of the accelerator must be maintained such that the throttle valve is held in an "increase" position until engine warm-up is completed.
The present invention tends to solve the above-mentioned problems, i.e., the difficulty of achieving reliable starting at low temperatures of engines equipped with a rotary throttle valve type carburetor and the inconvenience associated with such starting. An object of the present invention is to provide a carburetor which makes it possible to achieve reliable starting by means of an extremely simple operation that tends not to require any skill, and which also has a function that allows a smooth transition to ordinary operation. A further object of the present invention is to enable reliable low-temperature starting of an engine by holding the throttle valve in an air and fuel "increase" position by means of a simple operation that tends to require no skill.
In the present invention a rotary throttle valve type carburetor, which is constructed to solve the aforementioned problems, includes (i) a cylindrical throttle valve which is installed crosswise in the air intake passage of the carburetor main body, and which has a throttle orifice and a metering needle; (ii) a push spring which drives the throttle valve toward the idle position; (iii) a fuel nozzle which is installed on a central axial line of the throttle valve and which opens into the throttle orifice, and into which the metering needle is inserted; and (iv) a constant-fuel chamber which holds fuel that is fed out from the fuel nozzle. The rotary throttle valve type carburetor of the present invention controls the air and fuel flow rates within it by turning a lever in accordance with the operation of the accelerator. Rotation of the lever which is mounted on the tip end of a valve shaft that protrudes from the throttle valve to the outside of the carburetor main body, causes the throttle valve to move along its central axial line while rotating.
A valve opening mechanism is also provided. The valve operating mechanism includes (i) a cam part that causes the lever to turn from the idle position to a position which slightly increases the amount of air and fuel supplied, and (ii) a return spring. The cam part of the valve-opening mechanism is manually moved from an inoperative position to an operative position where said cam part engages the lever in the idle position and places the throttle valve in the "increase" position. The cam part is fixed in the operative position by the spring force of the push spring. The cam part is returned to the inoperative position by the return spring when the lever is released.
As a result, the throttle valve can be moved to a prescribed "increase" position by means of an extremely simple operation which tends to require no skill, i.e., manual movement of the cam part. Furthermore, the throttle valve is held in the "increase" position even if the hand is removed, so that reliable starting is possible. Moreover, when the lever is turned by the operation of the accelerator, the lever is released from the cam part so that a smooth transition to normal operation can be achieved.
Furthermore, if an anchoring means is provided starting is made even more reliable. The anchoring means engages the lever and holds the throttle valve in the "increase" position when the cam part is moved into the operative position, and adds to the spring force of the push spring so that the cam part is fastened in a stable manner in the operative position.
In cases where a stroke regulating means is provided, starting is made even more reliable. The stroke regulating means causes the cam part to move a fixed distance between the inoperative position and the operative position which causes the throttle valve to be moved to a fixed "increased" position, thus making starting more reliable.